In non-spherical silica sols in which non-spherical silica fine particles are dispersed in a solvent, a chain shape, a moniliform shape, and a spheroidal shape are known as the shape of the non-spherical silica fine particles. Such non-spherical silica sols are used as, for example, various polishing agents.
As a process for producing a non-spherical silica sol containing irregular-shaped particles, Japanese Unexamined Patent Application Publication No. 1-317115 (PTL 1) discloses a process for producing a non-spherical silica sol containing a liquid medium and elongated-shaped non-spherical colloidal silica particles dispersed in the liquid medium, in which a ratio D1/D2 of a particle diameter (D1) measured by an image analysis method to a particle diameter (D2) measured by a nitrogen gas adsorption method is 5 or more, D1 is 40 to 500 millimicron, and the non-spherical colloidal silica particles each have a uniform thickness in a range of 5 to 40 millimicron measured by electron microscopy and have an extending direction only in one plane. This production process includes (a) a step of adding a predetermined amount of an aqueous solution containing a water-soluble calcium salt, a magnesium salt, or the like to a predetermined colloidal aqueous solution of activated silicic acid, and mixing the solutions, (b) a step of further adding an alkali metal oxide, a water-soluble organic base, or a water-soluble silicate thereof so that a ratio SiO2/M2O (where M represents an atom of the alkali metal atom or a molecule of the organic base) is 20 to 200 on a molar ratio basis, and mixing the resulting mixture, and (c) a step of heating the mixture obtained in the previous step at 60° C. to 150° C. for 0.5 to 40 hours.
Japanese Patent No. 3441142 (PTL 4) proposes a polishing agent for a semiconductor wafer, the polishing agent being composed of a stable sol of silica, in which the number of colloidal silica particles having a major diameter of 7 to 1,000 nm and a minor-diameter/major-diameter ratio of 0.3 to 0.8, the major diameter and the minor diameter being determined by image analysis of an electron micrograph, accounts for 50% or more of the total number of particles.
Japanese Unexamined Patent Application Publication No. 7-118008 (PTL 5) discloses a process for producing an elongated-shaped non-spherical silica sol obtained by adding an aqueous solution of a water-soluble calcium salt, magnesium salt, or a mixture thereof to a colloidal aqueous solution of activated silicic acid, adding an alkaline substance to the resulting aqueous solution, heating a portion of the resulting mixture at 60° C. or higher to prepare a heel solution, preparing a feed solution composed of the remaining portion of the mixture, adding the feed solution to the heel solution while evaporating water during the addition so that the SiO2 concentration is concentrated to 6% to 30% by weight.
Japanese Unexamined Patent Application Publication No. 8-279480 (PTL 6) discloses a colloidal silica aqueous solution in which colloidal silica particles having a particle diameter of usually 4 to 1,000 nm, and preferably 7 to 500 nm are dispersed in an aqueous medium, and which has a SiO2 concentration of 0.5% to 50% by weight, and preferably 0.5% to 30% by weight. The colloidal silica aqueous solution is produced by, for example, (1) a method including neutralizing an aqueous alkali silicate solution with a mineral acid, adding an alkaline substance to the resulting solution, and aging the resulting mixture by heating, (2) a method including adding an alkaline substance to an activated silicic acid obtained by performing a cation exchange treatment of an aqueous alkali silicate solution, and aging the resulting mixture by heating, (3) a method including aging activated silicic acid by heating, the activated silicic acid being obtained by hydrolyzing an alkoxysilane such as ethyl silicate, or (4) a method including directly dispersing silica fine particles in an aqueous medium. It is described that the shape of the silica particles is a spherical shape, a distorted shape, a flat shape, a flaky shape, an elongated shape, a fibrous shape, or the like.
Japanese Unexamined Patent Application Publication No. 11-214338 (PTL 7) discloses a method for polishing a silicon wafer using a polishing material containing colloidal silica particles as a main component, the method being characterized in that colloidal silica particles having a major-diameter/minor-diameter ratio of 1.4 or more and obtained by allowing methyl silicate purified by distillation to react with water in methanol solvent using, as a catalyst, ammonia or ammonia and an ammonium salt are used.
International Publication No. WO00/15552 (PTL 8) describes a non-spherical silica sol containing spherical colloidal silica particles having an average particle diameter of 10 to 80 nm and metal-oxide-containing silica that joins the spherical colloidal silica particles to each other, in which a ratio D1/D2 of a particle diameter (D1) of the spherical colloidal silica particles measured by an image analysis method to a particle diameter (D2) thereof measured by a nitrogen adsorption method is 3 or more, D1 is 50 to 500 nm, and moniliform colloidal silica particles in which the spherical colloidal silica particles are linked in a row only in one plane are dispersed.
International Publication No. WO00/15552 also describes a process for producing the non-spherical silica sol, the process including (a) a step of adding an aqueous solution of a water-soluble metal salt to a predetermined aqueous colloidal solution of activated silicic acid or acidic non-spherical silica sol in an amount of 1% to 10% by weight on a metal oxide basis relative to SiO2 in the aqueous colloidal solution or the acidic non-spherical silica sol to prepare a mixed liquid 1 (b) a step of adding an acidic spherical silica sol having an average particle diameter of 10 to 80 nm and a pH of 2 to 6 to the mixed liquid 1 in such an amount that a ratio A/B (weight ratio) of a silica content (A) due to this acidic spherical silica sol to a silica content (B) due to this mixed liquid 1 is 5 to 100 and the total silica content (A+B) of a mixed liquid 2 obtained by mixing the acidic spherical silica sol and the mixed liquid 1 is 5% to 40% by weight on a SiO2 basis in the mixed liquid 2, and (c) adding an alkali metal hydroxide, a water-soluble organic base, or a water-soluble silicate to the resulting mixed liquid 2 so that the pH becomes 7 to 11, mixing, and heating the resulting mixture.
Japanese Unexamined Patent Application Publication No. 2001-11433 (PTL 9) describes a process for producing a moniliform non-spherical silica sol. In the process, an aqueous solution containing one or more water-soluble salt of a divalent or trivalent metal is added to an aqueous colloidal solution of activated silicic acid, the aqueous colloidal solution containing 0.5% to 10% by weight of SiO2 and a pH of 2 to 6 in such an amount that the content of a metal oxide (where, in the case of a salt of a divalent metal, the metal oxide is represented by MO, and in the case of a salt of trivalent metal, the metal oxide is represented by M2O3, however, M represents a divalent or trivalent metal atom and O represents an oxygen atom) is 1% to 10% by weight relative to SiO2 of the aqueous colloidal solution of activated silicic acid, and the resulting aqueous solution is mixed to prepare a mixed liquid (1). To the mixed liquid (1), an acidic spherical silica sol having an average particle diameter of 10 to 120 nm and a pH of 2 to 6 is added so that a ratio A/B (weight ratio) of a silica content (A) due to this acidic spherical silica sol to a silica content (B) due to this mixed liquid (1) is 5 to 100 and the total silica content (A+B) of a mixed liquid (2) obtained by mixing this acidic spherical silica sol and the mixed liquid (1) is 5% to 40% by weight on a SiO2 basis in the mixed liquid (2), and the resulting liquid is mixed. Furthermore, an alkali metal hydroxide or the like is added to the mixed liquid (2) so that the pH of the liquid becomes 7 to 11, and the resulting mixture is mixed to prepare a mixed liquid (3). The mixed liquid (3) is then heated at 100° C. to 200° C. for 0.5 to 50 hours.
Japanese Unexamined Patent Application Publication No. 2001-48520 (PTL 10) describes a process for producing a non-spherical silica sol in which elongated-shaped amorphous silica particles having an average diameter of 5 to 100 nm in a thickness direction and a length 15 to 50 times the average diameter, the average diameter and the thickness being measured by electron microscopy, are dispersed in a liquid dispersion medium. In the process, an alkyl silicate is hydrolyzed in the presence of an acid catalyst without using a solvent in a composition in which the silica concentration is in a range of 1 to 8 mol/L, the acid concentration is in a range of 0.0018 to 0.18 mol/L, and the water concentration is in a range of 2 to 30 mol/L. The reaction solution is then diluted with water so that the silica concentration is in a range of 0.2 to 15 mol/L. Subsequently, an alkali catalyst is added thereto so that the pH becomes 7 or more, and the resulting mixture is heated to carry out polymerization of silicic acid.
Japanese Unexamined Patent Application Publication No. 2001-150334 (PTL 11) describes a process for producing a non-spherical silica sol containing silica having a distorted shape. In the process, a salt of an alkaline earth metal such as Ca, Mg, or Ba is added to an acidic aqueous solution of activated silicic acid having a SiO2 concentration of 2% to 6% by weight, the acidic aqueous solution being obtained by performing an cation-removing treatment of an aqueous solution of an alkali metal silicate, such as water glass, in an amount of 100 to 1,500 ppm by weight on a basis of an oxide of the alkaline earth metal relative to SiO2 of the activated silicic acid. Furthermore, the same alkali substance is added in such an amount that a molar ratio SiO2/M2O (where M represents an alkali metal atom, NH4, or a quaternary ammonium group) in this liquid is 20 to 150 to prepare an initial heel solution. Similarly, an aqueous activated silicic acid solution having a SiO2 concentration of 2% to 6% by weight and a molar ratio SiO2/M2O (where M represents the same as the above) of 20 to 150 is prepared and used as a charge solution. The charge solution is added to the initial heel solution at 60° C. to 150° C. at a rate of 0.05 to 1.0 per hour on a basis of a weight ratio of charge solution SiO2/initial heel solution SiO2 while removing water from the liquid by evaporation (or without removing water).
Japanese Unexamined Patent Application Publication No. 2003-133267 (PTL 12) describes, as polishing particles which suppress dishing (excessive polishing) and with which a substrate surface can be evenly polished, polishing particles characterized by containing an irregular-shaped particle group in which two or more primary particles having an average particle diameter in a range of 5 to 300 nm are bonded to each other. It is described that, in particular, polishing particles in which the number of primary particles constituting the irregular-shaped particle group in the total number of primary particles in the polishing particles is in a range of 5% to 100% are effective.
Japanese Unexamined Patent Application Publication No. 2004-288732 (PTL 13) discloses a slurry for polishing a semiconductor, the slurry being characterized by containing non-spherical colloidal silica, an oxidizing agent, an organic acid, and water as a balance. In PTL 13, a slurry in which (major diameter/minor diameter) of the non-spherical colloidal silica is 1.2 to 5.0 has been proposed. Japanese Unexamined Patent Application Publication No. 2004-311652 (PTL 14) also discloses a similar non-spherical colloidal silica.
In addition, with regard to a silica-alumina-coated chain, non-spherical silica sol, Japanese Unexamined Patent Application Publication No. 2002-3212 (PTL 15) discloses a method for producing a silica-alumina-coated chain, non-spherical silica sol including (a) a step of adding a silicic acid solution to an aqueous solution of an alkali metal silicate containing 0.05% to 5.0% by weight of SiO2 so that SiO2/M2O (molar ratio, where M represents an alkali metal or a quaternary ammonium) in the resulting mixed liquid is 30 to 60; (b) a step of adding one or more metal compounds of a divalent to tetravalent metal before, during, or after the step of adding the silicic acid solution; (c) a step of maintaining the resulting mixed liquid at any temperature of 60° C. or higher for a certain period of time; (d) a step of adding a silicic acid solution to the reaction mixture again so that SiO2/M2O (molar ratio) in the reaction mixture is 60 to 200; and (e) a step of simultaneously adding an aqueous solution of an alkali metal silicate and an aqueous solution of an alkali aluminate to the reaction mixture on the alkali side.
As an example of silica-based fine particles having projection structures on the surfaces thereof, Japanese Unexamined Patent Application Publication No. 3-257010 (PTL 16) describes silica particles having continuous irregular projections with a size of 0.2 to 5 μm when observed with an electron microscope, the silica particles having an average particle diameter of 5 to 100 μm, a specific surface area of 20 m2/g or less determined by a BET method, and a pore volume of 0.1 mL/g or less.
Japanese Unexamined Patent Application Publication No. 2002-38049 (PTL 17) describes silica-based fine particles characterized in that substantially spherical and/or hemispherical projections are formed on the entire surfaces of seed particles, and the projections are bonded to the seed particles by a chemical bond. Furthermore, PTL 17 describes a process for producing silica-based fine particles including (A) a step of hydrolyzing and condensing a specific alkoxysilane compound to produce polyorganosiloxane particles; (B) a step of surface-treating the polyorganosiloxane particles with a surface adsorbent; and (C) a step of forming projections on the entire surface of the polyorganosiloxane particles that have been surface-treated in step (B) above using the alkoxysilane compound.
Japanese Unexamined Patent Application Publication No. 2004-35293 (PTL 18) discloses silica-based particles characterized in that substantially spherical and/or hemispherical projections are formed on the entire surfaces of seed particles, the projections are bonded to the seed particles by a chemical bond, and compression moduli of the seed particles and projections under 10% compression are different from each other.
However, the particles described in Japanese Unexamined Patent Application Publication No. 3-257010 (PTL 16) include only silica particles having an average particle diameter of 5 to 100 μm. Japanese Unexamined Patent Application Publication No. 2002-38049 (PTL 17) discloses only silica-based particles having an average particle diameter substantially in a range of 0.5 to 30 μm. Similarly, Japanese Unexamined Patent Application Publication No. 2004-35293 (PTL 18) also discloses only such particles.